This invention relates to a process for producing an improved aqueous emulsion coating composition which can form film having excellent resistance to chemicals and water.
The use of an organic solvent type coating composition may be restricted hereafter in order to prevent environmental pollution. Therefore, the development of an aqueous coating composition not containing an organic solvent has been demanded.
In the prior art, an aqueous emulsion type coating composition not containing an organic solvent has been prepared. The use of the aqueous coating composition is preferred in respect to preventing the environmental pollution, because no organic solvent is used in a process for preparing the aqueous coating composition. Also, no special facilities such as, solvent-removing means are necessary when using the aqueous coating composition. A variety of aqueous emulsion type coating compositions are used in practice for the foregoing reasons.
However, aqueous emulsion type coating compositions are inferior to the organic solvent type coating composition in resistance to chemicals and particularly resistance to water. A non-soap type emulsion coating composition and a low soap concentration emulsion coating composition have been developed for overcoming the above disadvantages. However, the compositions of this type still have the above mentioned disadvantages, so they are neither usable outdoors nor as a top coating.
A process which comprises coating a composition containing a radiation-sensitive prepolymer, such as an unsaturated polyester on a substrate, and then irradiating the resulting coating by means of ionizing radiation has also been developed. In practicing such process, an electron-generating apparatus, a radiation-shielding apparatus and an apparatus for securing an inert atmosphere are necessary on the spot at which said composition is coated. However, setting these apparatuses on the spot is expensive, so irradiation of the coating by means of ionizing radiation in situ was impossible in practical use. In addition, since electron beam has low permeability, and tends to go straight, it was difficult to irradiate the resin coated on a curved surface or at a joint with an electron beam.
In the prior art, an aqueous emulsion type coating composition usually contained a stabilizer having hydroxy groups, such as hydroxyethyl cellulose or polyvinyl alcohol.
When the aqueous emulsion type coating composition containing said stabilizer was irradiated with an ionizing radiation, the viscosity of the emulsion increased. In some cases, the emulsion composition changed to a semisolid state, such as a gelatinous state or a pudding-like state. Neither such emulsion composition having high viscosity nor such emulsion composition in a semisolid state was usable as a coating composition.
In general, when an emulsion composition containing a crosslinkable resin was irradiated with an ionizing radiation, the crosslinkable resin molecules contained in each dispersed particle in the emulsion composition crosslinked to one another. That is, each particle in the emulsion composition gave rise to gelation, and became rigid. Therefore, the film-forming properties of the emulsion composition were lost. For the reasons mentioned above, the prior art was defective in that when the aqueous emulsion type coating composition was irradiated with ionizing radiation, the film-forming properties of the composition were lost.